How the priming sugar calculation works
When you bottle-condition beer, you add a measured amount of fermentable sugar just before capping. The small population of yeast still in the beer eats that sugar and releases carbon dioxide (CO₂). Because the bottle is sealed, the CO₂ dissolves into the beer and carbonates it. Add too little sugar and the beer is flat; add too much and you risk over-carbonation or, in the worst case, exploding bottles. This calculator finds the amount that hits your target.
The calculation has three parts. First it works out how much CO₂ is already dissolved in the beer (residual CO₂), which depends on the warmest temperature the beer reached after fermentation. Then it subtracts that from your target carbonation to find how much CO₂ you still need to generate. Finally it converts that CO₂ into grams of sugar.
The formula
Residual CO₂, in volumes, uses the Zahm & Nagel equation with temperature T in °F:
Residual CO₂ = 3.0378 − (0.050062 × T) + (0.00026555 × T²)
The priming sugar mass then follows the standard homebrew relationship, with volume in US gallons:
Sugar (g) = 15.195 × gallons × (target CO₂ − residual CO₂) ÷ S
The factor S adjusts for sugar type. Table sugar (sucrose) is 100% fermentable and is the reference (S = 1.00). Corn sugar (dextrose) is sold as a monohydrate that is about 91% sugar by weight, so S = 0.91, meaning you need a little more of it. Dry malt extract (DME) is only partly fermentable, roughly 68% (S = 0.68), so it takes about half again as much by weight.
Why beer temperature matters
CO₂ is more soluble in cold liquid, so a beer that stayed cold holds more residual CO₂ and needs less priming sugar. A beer that warmed up has already lost dissolved CO₂ and needs more. What matters is the highest temperature the beer reached after active fermentation, not the temperature at bottling. At 68 °F (20 °C) the residual is about 0.85 volumes; at 40 °F (4 °C) it rises to about 1.46 volumes.
Common target carbonation levels
- British ales, stouts, porters: 1.5–2.2 volumes — low, soft carbonation
- American ales, IPAs, ambers: 2.2–2.7 volumes — the everyday default is around 2.4
- Wheat beers, Belgians, saisons: 3.0–4.5 volumes — high carbonation (use heavy bottles)
- Lagers, pilsners: 2.4–2.9 volumes
As a benchmark, a 5-gallon (19 L) batch of American ale at 2.4 volumes with beer at 68 °F needs about 128 g of corn sugar or 117 g of table sugar.